Especially, the field of the application is RFID systems operating at 13.56 MHz which is usually referred to as high frequency RFID, HF RFID. Communication in these systems is achieved by magnetic or inductive coupling between an RFID reader coil and an RFID tag coil. Traditionally, RFID tags are passive devices. They are usually composed of an integrated circuit, IC, and a coil which is attached to its terminals. Together with an integrated capacitor, this coil forms a resonant circuit with a resonance close to 13.56 MHz. Since passive RFID tags do not contain a power source, the reader magnetic field is not only used to send data to a tag, but also provides a power supply to the RFID tag. However, the tag cannot reply to a reader by an active transmission since the induced power is not sufficient. Therefore, a state of the art passive RFID tag communicates to RFID readers by switching on and off a load connected in parallel to its coil by means of a so-called modulator switch. The term load modulation is usually used for this kind of communication. A standard that is commonly used in this communication is ISO/IEC 14443. The operating range of HF RFID systems varies from a few cm up to 1 m, depending on the protocol used and power emitted by the reader. The operating range of systems according to ISO 14443 standard, which are the most widely used HF RFID systems, is up to 10 cm using credit card size RFID tags. Such systems are used for contactless payment, ticketing, access control and similar applications.
The physical properties of HF RFID systems are such that the range in which conventional tags are supplied and the range in which the reader can detect a signal produced by load modulation are approximately the same. This means that the range of a tag to reader communication cannot be significantly improved even if an external power supply is provided to an RFID tag.
There is a strong demand on the market to implement RFID tags with very small dimensions. A typical application is integrating RFID tag functionality in a mobile phone, either as an imbedded application having a small antenna or in a card which is inserted in the phone, e.g. a micro SD card or a SIM card. In these applications, the size of the RFID tag antenna is so small that the use of a passive RFID tag IC results in the best case in a very short operating range. As mentioned before, the solution to increase the operating range is to provide a power supply and replace load modulation by another technology that increases the communication range in the direction tag to reader. One possibility is to replace load modulation by an active transmission. Active Load Modulation, ALM, is the term used for this type of modulation.
To generate a tag reply, ALM devices generate a signal synchronous to the incoming field frequency received from a reader. The signal to be transmitted is generated by means of a digital amplitude modulation, also known as amplitude shift keying, using a subcarrier signal. Two different types of transmission are possible: According to a first type, transmission is only active during the time that the modulator switch would be turned on in a passive load modulation device. In the case of ISO 14443 standard, this is equivalent to a first half of a subcarrier period. In the second type transmission is active in both parts of the tag reply, the part when the modulator switch of a passive load modulation device would be turned on and the part when it would be turned off. During the time the modulator switch would be turned off, a signal is sent in opposite phase compared to the signal received from the reader. In the ISO/IEC 14443 standard, this is equivalent to sending a signal in the opposite phase during the second half of the subcarrier period.
Both transmission techniques listed above are described, for example, in the ISO contribution tf2n723_Active_Transmission_PICC_to_PCD. Different terminology is used to refer to the two modes. In the present application the expression AND TX mode is used for transmission only during the first half of the subcarrier period, i.e. the first type described above, and the expression XOR TX mode is used for the second type described above.
XOR TX mode generates twice as much signal on an RFID reader as AND TX mode during the same period of time, because in XOR TX mode the signal is sent in opposite phase while in AND TX mode there is no transmission. Therefore, XOR TX mode is generally employed in applications where signals generated in AND TX mode are not strong enough or in case the operating range needs to be increased.
Both the AND and XOR TX modes can be used to implement the ISO/IEC 14443A with a bit rate of 106 kbps which uses Manchester coding.
Corresponding signals are depicted in FIG. 1. The first line shows a signal received from a reader device, the signal being denoted reader carrier. It has a carrier frequency which in the case of ISO 14443 is 13.56 MHz. This signal is also called the incoming field. The second line shows a transponder subcarrier signal which comprises data to be sent to the reader which has been coded with the required code according to ISO 14443, e.g. Manchester code, and modulated with the subcarrier signal. The third line depicts the signal which is transmitted in AND TX mode, wherein the carrier is transmitted only during impulses of the transponder subcarrier. The fourth line shows the signal which is transmitted in XOR TX mode in which the carrier is transmitted during impulses of the transponder subcarrier marked with 1 and a carrier with opposite phase is transmitted in pauses between two impulses of the transponder subcarrier marked with 2 and hatched in the opposite direction when compared with the parts marked with 1.
The above explanation also applies to near-field communication, NFC, technology, specifically for passive peer-to-peer communication mode and for card emulation mode which re-use the ISO 14443 standard. In NFC technology, the term target is used instead of transponder or tag.
FIG. 2 shows a block diagram of a typical state of the art ALM device which can be a transponder or an NFC controller. The main building blocks are an antenna L connected to a matching circuit MC and an IC. The IC incorporates a receiver RX with a gain stage, a demodulator, a clock extractor and a data slicer functionality, a phase-locked loop, PLL, block PLB with a phase detector, a loop filter and a voltage-controlled oscillator, VCO, functionality, a control circuit CTL and a transmitter TX, with a regulator and a driver functionality. The control circuit CTL can be, for instance, the complete digital circuit of an NFC controller.
As known to those skilled in the art, the depicted ALM device operates basically according to the following: A signal sent by a reader is detected by the antenna L and filtered in the matching circuit MC. It is passed to the receiver RX which demodulates the data and extracts a clock signal therefrom. Both signals are propagated to the control circuit CTL. During reception, the PLL inside the PLL block PLB is locked to the extracted clock. When the transmitter TX is active for sending data to the reader, the PLL loop is open and the VCO continues to operate with the phase which was established during receiving. The resulting PLL clock is provided to the transmitter TX and the control circuit CTL. The control circuit generates a subcarrier signal by dividing the PLL clock and modulates data to be sent with the subcarrier. The resulting subcarrier data is propagated to the transmitter TX which therefrom generates signals to be transmitted using the PLL clock. Tor transmission these signals are passed through the matching circuit MC to the antenna L.
It has turned out that the known solutions of ALM devices using XOR TX mode for implementing the ISO 14443A 106 kbps protocol have the problem that at the end of the counter-phase carrier transmission the strong modulation signal on the reader antenna, caused by the transponder active transmission, does not stop immediately. This entails a demodulator or data slicer on the reader side to produce more pulses than there actually are. In the worst case, this will cause an error in the data frame and the communication will need to be restarted, or else the frame will need to be resent if some kind of error handling is implemented. However, because the error is caused systematically, it is hard to establish a reliable communication and the user possibly needs to re-align the transponder or mobile phone with integrated transponder closer or further away from the reader antenna.
FIG. 3 shows an oscilloscope screenshot of an RFID communication according to the state of the art which has the above mentioned problem. The screenshot shows a detail of a transponder frame in reply to a reader command at the point in time where the carrier transmission is stopped which in this case is after there is no subcarrier pulse for nine carrier clocks. In the first line the screenshot is showing the transmit signal of the transponder which is using ALM and XOR TX mode for the case of ISO 14443A 106 kbps. The second line shows an analog received signal on the reader side. The third line shows the corresponding digitized signal on the reader side. The fourth line shows the RF field between the reader and the tag measured with a so-called spy coil.
One can see that although the transponder transmission in the first line is already stopped at a point in time ta, the analog and digital signal in second and third lines are still ringing which causes an additional pulse P in the digital signal on the output of a data slicer of the reader. Said unintended additional pulse P may cause an error in the receipt of data on the side of the reader.
There is accordingly a need in the art to provide a method and a transmitter circuit for communication using active load modulation in RFID systems which avoid errors on the side of a reader caused by XOR TX mode active load modulation.